(a) Field of the Invention
The present invention relates to a switch controller controlling switching operation, a switch control method, and a power supply including the switch controller.
(b) Description of the Related Art
FIG. 1 shows a power supply using a buck converter. In FIG. 1, a load of the power supply is illustrated as an LED string formed of a plurality of LEDs. In FIG. 1, a rectifying bride is realized as a bridge diode 1.
As shown in FIG. 1, AC power is rectified through the bridge diode 1 in the LED light emission diode. The bridge diode 1 full-wave rectifies the input AC power. A rectified voltage, that is, an input voltage is supplied to an inductor 2, and the inductor 2 supplies a driving current to the LED string according to operation of a switch S. A switching unit 3 including the power switch S controls switching operation of the power switch S.
When the power switch S is in the turn-on state, an LED current flowing to the LED string is increased, and when the power switch S is in the turn-off state, the LED current is decreased. When the input voltage is a sine wave and the power switch S is peak-controlled, a duty of the power switch S is controlled to be inversely proportional to the input voltage. In addition, the duty of the power switch S is proportional to a load.
FIG. 2 shows an input voltage, a duty, and an LED current of the power supply in an ideal case.
As shown in FIG. 2, the duty is controlled to be inversely proportional to the input voltage, and the LED current is changed in accordance with the input voltage. However, a duty and an LED current in an actual case are different from the duty and the LED current of FIG. 2.
FIG. 3 shows an input voltage, a duty, and an LED current of the power supply in an actual case.
In the actual case, the duty cannot be shorter than the minimum on-time due to a leading edge blanking period and a propagation delay. Thus, when the input voltage exceeds a predetermined threshold value as shown in FIG. 3, the duty is maintained constantly, that is, maintained with the minimum on-time.
Then, the LED current may be distorted rather than being a sine wave like the input voltage as shown in FIG. 3. Then, the power factor is decreased and the total harmony distortion (THD) is increased.
The distortion of the LED current causes an overcurrent, and the overcurrent may causes damage to an element. That is, the overcurrent flows to the LED string so that the LED element may be damaged.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.